Major issues associated with design of prosthetic components include the need to reduce wear of the materials forming the components and the need to optimize their mechanical performance, including stability and range of motion. For instance, design of an acetabular component in a total hip replacement typically includes materials and geometry that permit adequate range of motion of the stem head in the component while at the same time reducing tendency of the head to dislocate from the component and reducing wear of the materials forming the prosthesis.
Bipolar prostheses are a nonlimiting case in point. Bipolar prostheses generally provide two bearing surfaces between the (artificial) femoral head and the acetabulum. The first bearing surface is the typically smooth outer surface of the bipolar prosthesis shell, which may be formed of metal, ceramic material or as otherwise desired. A liner, conventionally formed of polyethylene material such as ultra high molecular weight polyethylene, may be adapted to fit tightly within the shell and provide an inner bearing surface which receives and cooperates with the femoral head in an articulating relationship to track and accommodate the relative movement between the femur and the acetabulum.
This dual bearing surface design is often indicated for patients whose acetabulae are relatively healthy and able to accommodate a prosthetic proxy for the anatomical femoral head. Bipolar prostheses leverage the theory that wear and erosion of the acetabulum will be reduced where articulating motion is absorbed by two bearing surfaces rather than one, i.e., relative movement between the shell outer surface and the acetabulum on the one hand and between the femoral stem head and the prosthesis liner on the other. The dual bearing surface thus distributes shear forces between the inner and outer bearings in order to spare the acetabular surface from wear and erosion. Additionally, acetabular wear is diminished through reduction of relative motion between the acetabular anatomy and the outer surface of the prosthesis because the inner bearing formed by the liner against the femoral stem head absorbs some of the motion. Moreover, the dual bearing surfaces typically provide greater range of motion than provided by either unipolar designs or conventional total hip arthoroplasty.
Even though bipolar prostheses are well beyond the first generation of design and implementation, a number of issues remain. These include (1) the potential for dislocation of the femoral stem head relative to the shell/liner; and (2) polyethylene wear issues.
The dislocation problem is exacerbated because dislocations often require surgical intervention to reestablish the prosthetic/hip joint. Accordingly, conventional bipolar designs and other hip prosthesis designs often feature a snug fit in which the stem head is captured in the liner. Such designs include a reduced diameter liner opening through which the stem head is forced before installation of the shell/liner. Other designs feature retention rings or locking rings. However, overemphasis on dislocation prevention can reduce range of motion between the femoral stem and the shell/liner. Additionally, previous designs are limited by the extent to which the polyethylene can “stretch” to accommodate the stem head in a capture fit. Some previous locking ring designs in bipolar prostheses have accommodated the issue by including a split in the annulus formed by the locking ring so that the diameter of the locking ring may be expanded for penetration of the stem head before installation of the prosthesis, and contracted again for installation in a manner that seeks to effectively capture the stem head and thus prevent dislocation. Split rings, however, present their own sets of issues, including potential to expand and contract in diameter even after installed which can in turn introduce polyethylene wear issues in addition to unacceptable “pistoning” of the stem head in the liner with potential polyethylene wear and stability complications.
Polyethylene wear has been recognized as an undesirable effect sometimes caused by articulating and non-articulating relative motion between a polyethylene surface and another surface such as, for instance, a metal surface such as that of a bearing in a hip prosthesis. One reason that polyethylene wear is considered serious is that even a very small volume of polyethylene debris actually includes a great number of polyethylene particles. For instance, one cubic millimeter of polyethylene wear debris can include up to ten trillion polyethylene particles. These particles are believed to cause osteolysis when they escape into nearby bone and tissue. Such polyethylene wear can occur between surfaces which articulate relative to one another and also between surfaces which are intended not to articulate but only engage only in so-called “micromotion.”
Various approaches have been adopted in order to reduce generation of polyethylene wear debris. These include precise and specialized finishing and treatment of the polyethylene and the metal or other surfaces which bear against the polyethylene. They also include measures to enhance wear performance of the polyethylene such as irradiation and other processes employed to increase cross-linking or other material properties in the polyethylene. While cross-linking is advantageous to enhance polyethylene wear performance, it can also reduce mechanical and physical properties of the polyethylene such as, for instance, yield strength, tensile strength, elongation and impact strength. Accordingly, while cross-linking measures are useful for reducing wear of polyethylene in bipolar prostheses, polyethylene liners and locking rings formed of cross-linked polyethylene can suffer reduced ability to capture and retain a femoral stem head in order to prevent dislocation.